Head
Head and neck anatomy focuses on the structures of the head and neck of the human body, including the brain, bones, muscles, blood vessels, nerves, glands, nose, mouth, teeth, tongue, and throat. It is an area frequently studied in depth by surgeons, dentists, dental technicians, and speech language pathologists. Musculoskeletal system The head is positioned upon the superior portion of the vertebral column, attaching the skull upon C-1, (the atlas). The skeletal section of the head and neck forms the superior segment of the axial skeleton and comprises skull, hyoid bone, auditory ossicles, and cervical spine. The skull can be further subdivided into: * (a) cranium, (8 bones: frontal, 2-parietal, occipital, 2-temporal, sphenoid, ethmoid), and * (b) facial bones, (14 bones: 2-zygomatic, 2-maxillary, 2-palatine, 2-nasal, 2-lacrimal, vomer, 2-inferior conchae, mandible). As the fetus develops, the facial bones usually form into pairs, and then fuse together. As the cranium fuses, sutures are formed that resemble stitching between bone plates. In a newborn, the junction of the paritial bones with the frontal and occipital bones, form the anterior (front) and posterior (back) fontanelle, or soft spots. The separation of the cranial bone plates at time of birth facilitate passage of the head of the fetus through the mother's birth canal, or pelvic girdle. The parietial bones, and occipital bone can overlap each other in the birth canal, and form the unusual looking "cone head" appearance in a newborn when delivered in a natural, or vaginal, delivery. The occipital bone articulates with the atlas near the foramen magnum. The atlas articulates with the occipital condyle superiority and the axis inferiority. The spinal cord passes through the foramen magnum providing continuity for the central nervous system (CNS). Articulation (anatomy) of the neck includes: flexion, extension, hyperextension (nodding yes), and rotation (shaking head no). Circulatory system Blood circulates from the upper systemic loop originating at the aortic arch, and includes: the brachiocephalic artery, left common carotid and left subclavian artery. The head and neck are emptied of blood by the subclavian vein and jugular vein. edit Blood supply Right side of neck dissection showing the brachiocephalic, right common carotid artery and its branches The brachiocephalic artery or trunk is the first and largest artery that branches to form the right common carotid artery and the right subclavian artery. This artery provides blood to the right upper chest, right arm, neck, and head, through a branch called right vertebral artery. The right and left vertebral artery feed into the basilar artery and upward to the Posterior cerebral artery, which provides most of the brain with oxygenated blood. The posterior cerebral artery and the posterior communicating artery are within the circle of Willis. The left common carotid artery divides to form the: internal carotid artery (ICA) and an external carotid artery (ECA). The ICA supplies the brain. The ECA supplies the neck and face. The left subclavian artery and the right subclavian artery, one on each side of the body form the internal thoracic artery, the vertebral artery, the thyrocervical trunk, and the costocervical trunk. The subclavian becomes the axiliary artery at the lateral border of the first rib. The left subclavian artery also provides blood to the left upper chest and left arm. Blood-brain barrier The Blood-brain barrier (BBB) is semi-permeable membrane that controls the capillary leak potential of the circulatory system. In most parts of the body, the smallest blood vessels, called capillaries, are lined with endothelial cells. Endothelial tissue has small spaces between each individual cell so substances can move readily between the inside and the outside of the vessel. However, in the brain, the endothelial cells fit tightly together to create a tight junction and substances cannot pass out of the bloodstream. Some molecules, such as glucose, are transported out of the blood by active transport. Specialized glial cells called astrocytes form a tight junction or protective barrier around brain blood vessels and may be important in the development of the BBB. Astrocytes may be also be responsible for transporting ions (electrolytes) from the brain to the blood. Blood return Blood from the brain and neck flows from: (1) within the cranium via the internal jugular veins, a continuation of the sigmoid sinuses. The right and left external jugular veins drain from the parotid glands, facial muscles, scalp into the subclavian veins. The right and left vertebral veins drain the vertebrae and muscles into the right subclavian vein and into the superior vena cava, into the right atrium of the heart. Lymphatic system The lymphatic system drains the head and neck of excess interstitial fluid via lymph vessels or capillaries, equally into the right lymphatic duct and the thoracic duct. Lymph nodes line the cervical spine and neck regions as well as along the face and jaw. The tonsils also are lymphatic tissue and help mediate the ingestion of pathogens. Tonsils in humans include, from superior to inferior: nasopharyngeal tonsils (also known as adenoids), palatine tonsils, and lingual tonsils. Together this set of lymphatic tissue is called the tonsillar ring or Waldeyer's ring. Oral cavity The mouth, also called the (oral cavity) or buccal cavity is the entranceway into the digestive system containing both primary and accessory organs of digestion. The mouth is designed to support chewing, (mastication) and swallowing, (deglutition), and speech (phonation). Two rows of teeth are supported by facial bones of the skull, the maxilla above and the mandible below. Teeth are surrounded by gingiva, or gums, part of the periodontium, support tissue of oral cavity protection. In addition to the teeth, other structures that aid chewing are the lips, cheeks, tongue, hard palate, soft palate, and floor of the mouth. Teeth Humans normally will produce two sets of teeth called primary dentition, or deciduous teeth, and secondary dentition, or permanent teeth. A tooth is the toughest known substance in the body exceeding bones in density and strength. Tooth enamel lends great strength to the tooth structure. The formation of a developing tooth includes the process of dentin formation, (see: Dentinogenesis) and enamel formation, (see: amelogenesis. As the tooth breaks through the gum into the mouth, the process is called eruption. The formation of teeth begins in early fetal development and goes through six stages: * (1) initiation stage, 6th - 7th week * (2) bud stage, 8th wk * (3) cap stage, 9th-10 wk * (4) bell stage, 11th-12th wk * (5) apposition * (6) maturation stage Tooth enamel is white initially but is susceptible to stains from coffee and cigarette usage. A tooth sits in a specialized socket called gomphosis. The tooth is held in location by a periodontal ligament, with the assistance of cementum. The white visible part of a tooth is called the crown. The rounded upper projections of the back teeth are cusps. The hard white exterior covering of the tooth is the enamel. As the tooth tapers below the gumline, the neck is formed. Below the neck, holding the tooth into the bone, is the root of the tooth. The inner portions of the tooth consist of the dentin, a bonelike tissue, and the pulp. The pulp is a soft tissue area containing the nerve and blood vessels to nourish and protect the tooth, located within the pulp cavity. There are various tooth shapes for different jobs. For example, when chewing, the upper teeth work together with the lower teeth of the same shape to bite, chew, and tear food. The names of these teeth are: * (1) Incisors, there are eight incisors located in the front of the mouth (four on the top and four on the bottom). They have sharp, chisel-shaped crowns that cut food. * (2) Cuspids. or canine tooth, the four cuspids are next to each incisor. Cuspids have a pointed edge to tear food. * (3) Premolars or (bicuspids), the four pairs of molars are located next to the cuspids. They crush and tear food. * (4) Molars, there are twelve molars, in sets of three, at the back of the mouth. They have wide surfaces that help to grind food. Adults have 32 permanent teeth, and children have 20 deciduous teeth. Salivary glands There are three sets of salivary glands: the parotid, the submandibular and the sublingual glands. The (exocrine) glands secrete saliva for proper mixing of food and provides enzymes to start chemical digestion. Saliva also helps to hold together the formed bolus which is swallowed after chewing. Saliva is composed of primarily of water, ions, salivary amylase, lysozymes, and trace amounts of urea. Periodontium The periodontium includes all of the support membranes of the dental structures surround and support the teeth such as the gums and the attachment surfaces and membranes. This includes epithelial tissues (epithelium), connective tissues, (ligaments and bone), muscle tissue and nervous tissue. Tongue The tongue is a specialized skeletal muscle that is specially adapted for the activities of speech, chewing, developing gustatory sense (taste) and swallowing. It is attached to the hyoid bone. Terms meaning tongue include "glosso" and "lingual." Mucosa The protective tissues of the oral cavity are continuous with the digestive tract are called mucosa or mucous membranes. They line the oral, nasal, and external auditory meatus, (ear), providing lubrication and protection against pathogens. This is a stratified squamous epithelium containing about three layers of cells. The lips are also protected by specialized sensory cells called Meissner's corpuscles. The cells of the inner oral cavity are called the buccal mucosa. Video thumb|300px|right Category:Anatomy and Physiology